Rigid thermoplastics such as ABS, glass-filled nylon 6, and polycarbonate are used for a variety of engineering and/or structural applications such as power tool housings, luggage handles, gear shifts, automotive pedals, etc. For many of these applications, there is a need to cover over or adjoin these articles to a soft, elastomeric layer. This layer serves to provide a comfortable, ergonomic feel, reduced slippage, and increased abrasion resistance. Ideally, this combination of "hard" and "soft" materials is produced through a thermoforming operation such as co-injection molding, multi-layer extrusion, or blow molding.
U.S. Pat. No. 5,154,979 is directed to a shaped article and a method for making such an article. The article consists of a base made of a first thermoplastic polymer. An attachment made of a second thermoplastic polymer is welded on top of the base. One or both of the first polymer and second polymer must contain from 5 to 75 percent by weight, based on the mixture, of an olefin homopolymer and/or copolymer. Typical examples would include an engineering thermoplastic power tool body with a handle grip made of an elastomer such as TPU, Santoprene, or the like. Unfortunately, the article disclosed in the '979 patent provides insufficient abrasion resistance in the elastomeric portion as well as being too hard for most uses which would require an elastomeric overlay.
Until now, most thermoplastic elastomers with the required so fitness (Shore A.ltoreq.80) to be useful as a soft covering, including propylene and ethylene-based EPDM polymers, styrenic block ter- or copolymers and plasticized PVC had insufficient adhesion to the rigid thermoplastic substrate unless affixed using separate adhesives or mechanical interlocking. Unfortunately, these soft TPEs also have ins efficient abrasion resistance for engineering applications. Although they have the desired Shore hardness, good adhesion and abrasion resistance, plasticized TPU (TPU blended with phthalate esters, such as DOP) is unacceptable due to the tendency of the plasticizer to migrate into the thermoplastic substrate and cause stress cracking, or to migrate into the thermoplastic/TPU interface, thus weakening adhesion.